The present invention relates to a method and a device for multizone capacitive detection. The field of the invention is more particularly but non-limitatively that of capacitive detection systems and touch and contactless human-machine interfaces.
Many communications and work appliances use touch or contactless measurement interfaces as a human-machine interface for entering commands. These interfaces can in particular be in the form of pads or touch screens. They are found for example in mobile phones, smart phones, touch-screen computers, tablets, pads, PCs, mice, touch screens and widescreens, etc.
These interfaces frequently use capacitive technologies. The measurement surface is equipped with conductive electrodes linked to electronic means which make it possible to measure the variation in the capacitances produced between electrodes and the object to be detected in order to carry out a command.
It is possible to produce transparent electrodes, which make it possible to superimpose an interface on a display screen, for example of a smart phone.
The majority of these interfaces are touch interfaces, i.e. they can detect the contact of one or more object(s) of interest or command (such as fingers or a stylus) with the surface of the interface.
Increasingly, gestural or contactless interfaces are being developed that are capable of detecting command objects at a greater distance from the interface, without contact with the surface.
The development of contactless interfaces requires the implementation of highly sensitive capacitive measurement techniques and the provision of a high degree of protection against environmental interference. In fact, the capacitance created between capacitive measurement electrodes of the interface and command objects is inversely proportional to the distance between them.
Document FR 2 756 048 by Rozière is known, which discloses a capacitive measurement method which makes it possible to measure the capacitance and the distance between a plurality of independent electrodes and an object in proximity.
This technique makes it possible to obtain measurements of capacitance between the electrodes and the objects with a high resolution and sensitivity, allowing the detection for example of a finger at a distance of several centimetres or even ten centimetres. The detection can be carried out in three-dimensional space, but also on a surface, known as a measurement surface.
Generally, the command object can be considered to be at a reference electrical potential such as an exposed conductive part or earth.
In the majority of capacitive detection techniques, the electrodes are polarized at an excitation voltage. A capacitive coupling is therefore measured between these polarized electrodes and the object at the reference potential.
In the case of a portable appliance such as a smart phone or a tablet, the electrical circuit comprises a reference potential or an internal earth which is floating with respect to earth, since it is supplied by a battery. However, through the effect of the capacitive couplings with its environment (in particular when it is held in a user's hand), this internal reference potential is set to earth or at least to the potential of the user's body. Thus, when the other hand comes close to the measurement electrodes, it is “seen” as being substantially at the internal reference potential or at the earth potential of the device.
There is an interest in the development of portable appliances, such as smart phones or tablets, which contain sensitive surfaces provided with measurement electrodes in places other than the display screen. Such appliances can be for example equipped with electrodes on the opposite face to the screen and/or on the sides, so as to detect additional items of information on their environment, the manner in which they are held, etc.
The problem arising in this case is that due to the floating nature of the installed electronics with respect to earth, if measurement electrodes polarized at an excitation voltage are in electrical contact or strongly coupled (capacitively) with for example the user's hand holding the device, the entire body of the user is “seen” by the electronics as substantially at this excitation potential. In this case the command object which is for example his other hand is also “seen” as being substantially at the excitation potential of the electrodes. The capacitive coupling is then zero or very weak and the object is not detected, or only at a short distance.
Capacitive detection techniques are also used to equip systems such as robots or mobile medical imaging appliances (scanners, etc.) in order to make them sensitive to their environment. The principle is the same: the capacitive coupling is measured between capacitive electrodes polarized at an excitation voltage and objects of the environment assumed to be at a reference electrical potential to an exposed conductive part or to earth.
When two objects equipped with capacitive electrodes are displaced relative to each other, there is a risk that they may not detect one another, because if the measurement electrodes are at the same potential no capacitive coupling is established between them. Furthermore, this is the guard principle which is used in the majority of capacitive detection systems: the electrodes are protected from undesirable environmental influences (parasitic capacitances) by having conductive surfaces at the same excitation potential in proximity.
A subject of the present invention is to propose a method for capacitive detection and a device which make it possible to overcome these drawbacks of the prior art.
Another subject of the present invention is to propose a capacitive detection method and device making it possible to equip electrically floating appliances with respect to a general exposed conductive part or to earth with capacitive detection electrodes capable of detecting the approach of objects of interest such that the measurements are not, or only slightly, affected by strong electrical couplings of certain electrodes with the exposed conductive part or earth.
Another subject of the present invention is to propose a method for capacitive detection and a device which makes it possible to equip portable appliances such as smart phones or tablets with capacitive detection electrodes on a plurality of faces such that the measurements are not, or only slightly, affected by strong electrical couplings of certain electrodes with the exposed conductive part or earth.
Another subject of the present invention is to propose a method for capacitive detection and a device which makes it possible to equip a plurality of appliances with capacitive detection electrodes such that these appliances are also capable of detecting each other.